Functions to calculate luminance of colors and contrast ratio of colors in SASS

luminance.scss

// Calulates power with integer exponent
@function pow($number, $exponent) {
  $value: 1;

  @if $exponent > 0 {
    @for $i from 1 through $exponent {
      $value: $value * $number;
    }
  } @else if $exponent < 0 {
    @for $i from 1 through -$exponent {
      $value: $value / $number;
    }
  }

  @return $value;
}

// Calculates nth root with iteration
@function nth-root($value, $n) {
  $precision: 1e-10;
  $previous: 0;
  $current: $value;

  @while abs($current - $previous) > $precision {
    $previous: $current;
    $current: 1 / $n * (($n - 1) * $previous + ($value / pow($previous, $n - 1)));
  }

  @return $current;
}

// Calculates luminance of a color based on the WCAG2 standard
// Luminance calculation based on: https://www.w3.org/TR/WCAG20/#relativeluminancedef
@function luminance($color) {
  $R: red($color) / 255;
  $G: green($color) / 255;
  $B: blue($color) / 255;

  @if $R <= 0.03928 {
    $R: $R / 12.92;
  } @else {
    $R: nth-root(pow($R + 0.055 / 1.055, 24), 10);
  }

  @if $G <= 0.03928 {
    $G: $G / 12.92;
  } @else {
    $G: nth-root(pow($G + 0.055 / 1.055, 24), 10);
  }

  @if $B <= 0.03928 {
    $B: $B / 12.92;
  } @else {
    $B: nth-root(pow($B + 0.055 / 1.055, 24), 10);
  }

  @return $R * 0.2126 + $G * 0.7152 + $B * 0.0722;
}

// Calculates the contrast of two colors based on the WCAG2 standard
// Constrast calculation based on: https://www.w3.org/TR/WCAG20/#contrast-ratiodef
@function contrast($color1, $color2) {
  @if luminance($color1) > luminance($color2) {
    @return (luminance($color1) + 0.05) / (luminance($color2) + 0.05);
  } @else {
    @return (luminance($color2) + 0.05) / (luminance($color1) + 0.05);
  }
}